1. Field of the Invention
The present invention generally relates to an image forming device using an electrophotographic system such as a copier, a printer, a facsimile, or a multifunction machine thereof, and a fuser arranged therein, and more specifically, to a fuser and an image forming device in which a magnetic induction heating system is used.
2. Description of the Related Art
Conventionally, an image forming device such as a copier, a printer, and the like is well known in which a fuser of the electromagnetic induction heating system is used for the purpose of reducing the time necessary for starting the device so as to save energy. Such a device is disclosed in Japanese Laid-Open Patent Application Publication No. 2005-70376.
In Japanese Laid-Open Patent Application Publication No. 2005-70376, the fuser of the electromagnetic induction heating system comprises a heating roller as a heating member, auxiliary fuser roller (fuser roller), a fuser belt as a fuser member stretched between the heating roller and the auxiliary fuser roller, induction heating unit (induction heating means) as a magnetic flux generation unit opposing the heating roller across the fuser belt, a pressure roller contacting the auxiliary fuser roller across the fuser belt, or the like. The induction heating unit comprises a coil portion (excitation coil) extending in the width direction (perpendicular to the direction of delivering the recording medium), a core unit opposing the coil portion (excitation coil core), or the like.
The fuser belt is heated at a position opposing the induction heating unit. The heated fuser belt heats and fuses a toner image onto the recording medium delivered between the auxiliary fuser roller and the pressure roller. More specifically, alternating current of high frequency flows through the coil portion so as to form an alternating field around the coil portion resulting in an eddy current in the vicinity of the surface of the heating roller. When the eddy current is generated in the vicinity of the heating roller, Joule heat is generated due to an electric resistance of the heating roller itself. The fuser belt stretched on the heating roller is heated by the Joule heat.
In such a fuser of the electromagnetic induction heating system, the heating element is directly heated by the electromagnetic induction. Hence, efficiency of heat exchange is greater than the efficiency of other systems such as a thermal roller system (heater lamp heating system). Accordingly, it is well known that the surface temperature (fusing temperature) can be increased to a desired temperature with a small amount of energy in a short period of time.
In the conventional fuser as above described, when the fuser belt as the fuser member is off to the side (displacement) in the width direction, an edge of the fuser belt may fail to make contact with the heating roller as the heating member so that there is a possibility of an overincrease of the temperature of the end of the heating roller in the width direction.
The operations are described more specifically below.
In the conventional fuser, in order to prevent the fuser belt opposing the induction heating unit from being off to the side in the width direction (displacement), protrusions (displacement stoppers) may be provided at both edges in the width direction of the fuser belt on the inside surface thereof. In addition, an engaging portion contacting the protrusion may be provided at a flange press-fitted in each of ends of the heating roller. That is, even if one end of the fuser belt is about to be off to the side in the width direction, the protrusion provided at the other end in the width direction contacts the engaging portion of the heating roller configured as the stopper so as to prevent the fuser belt from moving in the width direction.
However, in the case where the protrusion of the fuser belt runs on the engaging portion of the heating roller (including a case where the protrusion is not completely on the engaging portion, but is almost so), the fuser belt is moved apart from the heating roller there. Accordingly, when the fuser belt fails to contact the heating roller, the temperature of the heating roller immediately overincreases at the end thereof in the width direction where the contact failure occurs. This is because the heat of the heating roller, which is generated by the electromagnetic induction, is not transferred to the fuser belt (not contacting with each other) so that the heat is accumulated inside the heating roller. When the temperature of an end of the heating roller in the width direction overincreases, the temperature of the corresponding flange press-fitted in the end of the heating roller becomes greater than its heat-resistant temperature so that the flange may be damaged.